Celebrity trainer Ron “Boss” Everline has a core exercise challenge for you, and spoiler: it’s much harder than it looks.

The founder of the Just Train fitness brand, whose famous clients have included Christina Milian, Kevin Hart, and Ne-Yo, among others, posted an Instagram video on Tuesday of him demoing what he dubs the “hardest core challenge I’ve done in a while.” It’s a one-legged, kneeling rotation move that requires a ton of balance, evident in the extremely focused expression he maintains throughout the video.

“Y’all gotta try this!!!” Everline writes in the caption before tagging fellow trainer Kasia Keranen (@kasiafit) and the account @realgame.athletics, which shared its own iteration of the challenge over the weekend. In the comments under Everline’s post, the two trainers, plus hundreds of others, chimed in.

“@justtrain as soon as I get off this plane I’m heading straight to the gym to show you how it’s done... 👊,” responded Keranen, and Danielle Gray (@Daniellegrayfit), an Equinox trainer, added “@kasiafit I feel the same way I wanna try and show him up. Maybe I’ll be texting @justtrainwhile DOING this exercise 😂😂.”

“I'm duplicating this tonight!!,” wrote @notyouraveragefitness, a personal trainer named Christopher Wilson, according to the bio.

As the move continues to spread across the ‘Gram, we chatted with two personal trainers to learn more about this core-slash-balance challenge, including what makes it so damn difficult, the specific skills and strength required to nail it, and beginner-friendly ways to work up the move.

There are several reasons this one-legged drill is so tough.

First, as Everline mentions, you need a high level of balance to pull it off, thanks to the kneeling stance. Balancing on one foot can be challenging in and of itself, but balancing on a knee—a much less stable base than the foot—is even more difficult. “Being on your knee is very unstable,” says Brewer.

This stance also makes the hip rotation component—which Everline demos in the second portion of the video—especially challenging as well. Thanks to the shorter distance between your hips and the ground in a kneeling versus standing position, you have to have even more strength and mobility in your hip flexors to lift your hips up—and then keep them up, Stephanie Mansour, Chicago-based certified personal trainer, tells SELF. “The hip flexor flexibility is really challenging,” she says.

On top of that, to do any portion of the move well, you need to activate essentially every major muscle in your core, including your rectus abdominis (what you think when you think abs), transverse abdominis (the deepest core muscle that wraps around your sides and spine), and internal and external obliques (muscles on the sides of your stomach), says Mansour. The hip rotation in particular is “a great oblique challenge,” says Brewer.

Lastly, though this move is core-centric, it does requires some strength in the multifidus (the thin muscle that runs along the spine), as well as the glutes and hamstrings, adds Mansour. In other words, it really works the core and lower body.

If you were to attempt this move on the reg, you’d feel several big benefits.

These benefits include increased hip mobility, improved strength in the core and hip flexors, and better balance, says Brewer.

Also, because this move is a single-leg exercise, it could also help reveal any discrepancies in balance and strength between the right and left sides of your body, adds Mansour. Muscle imbalances in particular can lead to injury over time if they’re severe enough and left unchecked, which is why doing single-leg exercises like this one can help.

Though you wouldn’t want to do this move as a set of reps, says Mansour, it would make for a fun challenge at the end of a workout, says Brewer.

Want to give the challenge a go? Here are Brewer and Mansour’s suggestions for safely working up to it.

Because this challenge, as mentioned, is quite advanced, here are more beginner-friendly ways to build up the requisite skills and strength. [Also important to note: Though the move, overall, is safe for most exercisers to attempt, if you have knee joint issues, it’s probably best to skip it, says Mansour, because of the pressure it could put on your kneecap.]

First, stand up alongside a pole or something you can lean on, says Brewer. Lean one half of your body against the pole and then lift your opposite leg up as high as you can and bend at the knee. If you can balance comfortably in this position, try performing hip circles with your elevated leg. “This is a good way to start opening up your hips,” says Brewer.

Once you feel comfortable with the standing hip circles, place a pad or mat on the floor (this will protect your kneecap, says Mansour) and try balancing on one knee while lifting the other knee out in front of your body. As you balance in this position (and all progressions of the position), make sure that the hip of your grounded leg is directly over that knee and that the calf is pointed straight back (not turned in or out) with the ankle directly behind the knee. This will protect your knee joint, says Mansour. For added stability, press the toes of your grounded foot into the floor, and raise your arms out to the side (don't place them on your hips) for extra balance help, says Mansour.

If you’re still wobbly here, you can lightly rest your fingertips on a couch, chair, or other nearby object. Use the fingers on the same side as your elevated leg, says Mansour, and make sure to keep your touch light; otherwise, you'll reduce the core challenge. You can also aid your balance by fixing your gaze on one spot on the floor, says Mansour, ideally a point that’s several feet in front of you.

When you feel comfortable and stable in this position, up the complexity by slowly lowering and lifting the elevated leg. Once you can do this for 5 to 8 consecutive seconds, progress the move by hinging your elevated leg out to the side and then back in. Start with a small range of motion, suggests Mansour, and consider placing a paper plate or other very light object atop your elevated quad. This will provide feedback on your balance, she explains. If the object falls off, you'll know you need to really work on your balance before progressing the move.

Once you’re comfortable with the leg hinges, you can increase the challenge by adding a light, weighted plate on top of your elevated quad (like Everline does in the video). Or you can attempt extending your elevated leg straight back behind you, as he also demos (removing the weight first, of course).

Whatever iteration of the move you attempt, remember: “you’re not looking for speed,” says Brewer. Rather, it’s about maintaining balance and good core activation as you perform slow, controlled movements.

Lastly, don’t be discouraged if you find this move extremely challenging and/or near impossible. After all, according to Everline, that’s exactly the point.